CMP brake question

[Phil Waterman]

Hi All


As you may remember a month ago I posted a question about Major/Minor brake adjustments on the larger CMPs at that time I wrote;


"The last three times I have done this three wheels RF, LF, LR all needed almost any nothing, but the Right Rear both A & B have needed much more. What I also noticed was that they turned much easier than all the others."


Here is the Right Rear B Adjuster as it was after adjustment


Screen Shot 2019-08-14 at 3.03.24 PM.jpg


Here is the Right Rear B Adjuster 62 miles later as it is today.


IMG_2584 (1).jpg


None of the other A or B adjusters have moved, so I think the problem is clear one adjuster is no longer staying put but is rotating back from the force of the return spring. Obviously this is not a state of affairs that can be left uncorrected.


Cheers Phil



So the question is do what do I do and in what order.

[Lynn Eades]

Hi Phil, Can you change out the backing plate?
If not, is the offending adjuster working its way off the (adjuster) bolt.
The bolt is usually staked on the outside of the snail. Try compressing the snail back further onto the bolt, with something like a valve spring compressor. If it moves back, peen the bolt?

[Grant Bowker]

Is it possible that the spring for the offending adjuster has either partially or fully cracked? This would cause loss of tension and also create blunt ends at the crack site instead of the ground flat ends of the spring. Possibly leading both to reduced tension and angled application of pressure. I've only seen one such cracked spring (that I recognized, maybe there were others) and it wasn't very obvious.
Just curious, do you notice any difference in force to rotate the two directions on that spring and compared to the other spring on the same wheel? You might have to take out the brake shoes to get enough rotation to notice and remove the effects of all parts other than the specific spring but you are probably getting close to doing that as you investigate.....

[rob love]

Could it just be the wind up of the spring caused the adjuster to back off? Solution would be to sparingly put a little oil between the backing plate and spring, and the spring and the adjuster cam.



The original was, as Lynn says, likely peened at the cam. The peen could be ground off (in situ...no need to remove the backing plate), the "snail" removed, and a suitable spring put in place. A cut open socket or piece of tubing could be C clamped into place to hold the cam in place, and a spot of mig weld put on to secure the cam. The heat would have to be kept to a minimum in order to not destroy the tensility of the spring.



The proper way would be t have the backing plate on the bench, and the cam would be put in place with a staking tool. We had a C-clamp for that purpose with the Iltis series of vehicles.

[Harry Moon]

If you don't have a different backing plate, grant Hopkins has NOS studs and springs.

[Phil Waterman]

Hi Lynn. Grant, Rob


Thanks for the input, you've added to the list of things I want to check.



1. The idea of the spring having lost tension, fits in with the earlier observation that this adjuster turns easier than the others. Next step will be to put a torque wrench on it and see if it really is turning easier.


2. The thought that the spring itself might be winding up as the adjustment is being made, had not occurred to me.



3. Confirming that the adjustment cam is not loose on the bolt is a big thing to rule out. If it is I can not think of the damage that it could do if it dropped off in side the drum. So I suspect that pulling the drum will be a necessity.


4. A broken or weaken spring from crack, rust, age, is I suspect the most likely cause. But what is the fix? Replace spring, or put a spacer under it to increase tension?


5. Replacing the backing plate is probably the hardest, as I have no spare and a used one is just as likely to have the same problem.


This is one of those brain teasers problems that not only do I want to fix but as important I want to understand the cause. Now that I have seen this happen on one adjuster where marking it has shown that it is actually moving. I will be marking all brake adjusters major and minor on the other trucks. Being able to look at a line on the part and know it changed is really nailed down this brake issue.


I'm looking at doing a couple of hundred mile convoy trip with the club in early September so nailing this down a being sure of the fix is important. Either than or carry a wrench and every time we stop slide under the truck and put the line back horizontal.



Will report back with more details and photos of what I find.


Cheers Phil


PS - While I was typing Harry provided a source for NOS replace part. Thanks Harry will follow up if that is the cause.

[rob love]

Never be afraid of pulling a wheel, or in the case of tracked vehicles, breaking track. In the long run, it's better to be thorough. If the problem is buried in the drum, you are just treading water trying to avoid the drum removal.

[Phil Waterman]

Hi All


Well after responding this morning, went out to the shop rolled under the truck and reset the adjustment this time using a torque wrench to see how much force was needed to adjust or back off. On this particular adjuster less than 2 ft.lbs. the adjuster on the other side was between 10-15 lbs depending on rotation direction. Something is up.


With that adjuster back at the correct point the brake peddle is right back up at the top of the stroke 1/2-1 inch of travel.


Drove the truck around the loop to the post office and back distance 7 miles pulled back into the shop and the marker line had again moved in the counter clockwise direction a visible amount.


Got my roll around drum puller down from the loft and put the truck up on jack stands. Tomorrow I'll pull the drum. As Rob suggest pulling the drum is the best way to be sure of what your dealing with. That is why I built a unit for pulling the drums on these rigs. That has a bolt plate that matches the studs and a lifting arm that is hydraulic so I can just roll the unit the truck level everything and undo the axle nuts and just roll the drum straight out. No damage to seal and more important do damage to me. Just can not man handle the heavy parts like I did years ago.


Cheers Phil

[Jacques Reed]

Originally Posted by Phil Waterman

Hi Lynn. Grant, Rob

Thanks for the input, you've added to the list of things I want to check.

1. The idea of the spring having lost tension, fits in with the earlier observation that this adjuster turns easier than the others. Next step will be to put a torque wrench on it and see if it really is turning easier.

2. The thought that the spring itself might be winding up as the adjustment is being made, had not occurred to me.

3. Confirming that the adjustment cam is not loose on the bolt is a big thing to rule out. If it is I can not think of the damage that it could do if it dropped off in side the drum. So I suspect that pulling the drum will be a necessity.

4. A broken or weaken spring from crack, rust, age, is I suspect the most likely cause. But what is the fix? Replace spring, or put a spacer under it to increase tension?

5. Replacing the backing plate is probably the hardest, as I have no spare and a used one is just as likely to have the same problem.

Cheers Phil

PS - While I was typing Harry provided a source for NOS replace part. Thanks Harry will follow up if that is the cause.

Hi Phil,

Just looking at a brake adjuster cam that I removed a long time ago from a F15-A rear backing plate. I doubt if the cam is turning on the bolt as the elongated hole on the cam would prevent that.

A broken spring could well be the problem as that is the reason I removed this adjuster cam. The spring in the picture is a new one I had made up. In the end I found two NOS rear backing plates and never proceeded with replacing this cam's spring and another cam than had a broken spring.

If the spring is not broken then the thought occurred to me, what if the cam is resting on the shoe just past the highest point, and the return spring is enough force to rotate the cam back towards the low point on the cam that has a weakened spring?

If the linings have substantial wear it is possible the cam would be close to the top of the cam at the contact point with the shoe.

If you have to remove the cam to replace a spring a method that worked for me was to carefully grind down the side of the flat on the bolt where it is splayed to hold it to the cam. I used a Dremel cut off disk to do it. Was going to then weld the cam back on to the top of the bolt but never had to do it thanks to finding the NOS plates.

Will look forward to hearing the solution to the problem.

Cheers,

[Tony Smith]

Or you could get a bit fancy?:

https://www.ebay.com/itm/39-48-FORD-...sAAOSwKIpWEWN1